Modelling the anisotropy of turbulence with the SWASH model
Heterogeneous roughness conditions in open channel flows
T. Bogaard
W.S.J. Uijttewaal – Mentor
M. Zijlema – Mentor
B.C. van Prooijen – Mentor
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Abstract
In this study the focus is on modelling turbulence anisotropy in open channel flows with the SWASH model. Turbulence anisotropy significantly influences the flow features of: channel flows with heterogeneous roughness conditions, curved open channel flows, compound channel flows with different floodplain depths, etc. The SWASH model is a non-hydrostatic wave-flow model, mainly used to predict the transformation of surface waves from offshore to the beach. For this study, adaptations were made to this SWASH model, in order to model turbulence anisotropy. Two different modelling approaches were used: RANS modelling and Large Eddy Simulation (LES). The SWASH model is extended with a non-linear k-? closure to the RANS equations, since the standard linear closure does not take turbulence anisotropy into account. A 3D subgrid model is implemented to perform LES. The performance of the LES code and the RANS model with the non-linear k-? closure is tested on two flow geometries: an open channel flow with homogeneous bottom roughness conditions and an open channel flow with parallel smooth to rough bed sections. Results of the RANS computations, for both horizontal homogeneous and non-homogeneous open channel flow, show good agreement with laboratory measurements of Muller and Studerus [13], Nezu and Rodi [17] and Wang and Cheng [32]. Although there is a number of closure constants involved with the non-linear k-? model, additional tuning of these coefficients was not necessary for this study: both the homogeneous and non-homogenous test case were simulated successfully using the standard values proposed by Speziale [25]. With its low computational costs and robustness, the non-linear k-? model appears to be a useful extension to the SWASH wave-flow model. LES results for horizontal uniform flow are validated with DNS data of Moser, Kim and Mansour [12]. Especially near the bed the LES results deviate from the DNS data. The mean velocity, as well as the transverse and vertical turbulence intensities, is seriously underestimated. The deviation from the DNS data is related to the use of non- periodic boundary conditions, the coarse grid resolution, the size of the computational domain and the amount of numerical dissipation that is involved. Since it is the bottom region where secondary currents are generated, the use of the present LES code for problems involving heterogeneous roughness is not appropriate.